Polyesters made from aromatic dicarboxylic acids and diphenolic compounds are particularly well known for their suitability for molding, extrusion and solvent casting applications. Among the many methods known for preparing such polyesters, the reaction of the dihalide of the dicarboxylic acid with the diphenol in the presence of an acid binding medium in the well known "interfacial" and solution methods are the most common. The reaction between a dialkyl ester of the dicarboxylic acid and a diphenolic compound is slow and requires long heating times because of the lack of reactivity of phenolic hydroxyls in general toward the dialkyl ester; moreover, polyesters formed in this manner are usually of low molecular weight and because of the long heating periods necessary, have a tendency to be badly degraded and discolored. It is further known that the slowness of the reaction and long heating times can be eliminated by using a diaryl ester instead of the dialkyl ester of the dicarboxylic acid. See, e.g., British Pat. No. 924,607.
The polyaryl esters produced by the transesterification and polycondensation of the diaryl esters of aromatic dicarboxylic acids and dihydric phenols have such high melting viscosities that the degree of polycondensation required for obtaining good mechanical properties cannot be reached in conventional stirred tank reactors. For the purpose of eliminating this disadvantage, it has been proposed heretofore to replace a part of the dihydric phenols with equivalent amounts of glycols. In U.S. Pat. No. 3,399,170 Blaschke et al. prepared polymers from a diaryl ester, a bisphenol and a glycol. However, this process has the disadvantages of poor reducibility and the necessity that the reactants be present in an exactly stoichiometric proportion with respect to each other.
The use of a "prepolymer" in preparing linear aromatic polyesters is not new. For example, Shatz et al. (U.S. Pat. No. 3,498,950) teach reacting a difunctional aliphatic modifier such as glycol with an excess of diacid halide followed by reacting the resulting "prepolymer" with a bisphenol. Schade et al. in U.S. Pat. Nos. 3,299,172 and 3,413,379, teach reacting diaryl esters of the aromatic dicarboxylic acids with dihydric phenols until the polyester produced has a melting viscosity above about 2000 poise and then adding a linear thermoplastic polyester consisting of terephthalic acid units, isophthalic acid units or mixtures thereof as well as residues of di-primary di-alcohols and then continuing the polycondensation until the product has a specific viscosity of about 0.7 or more. Schade et al. observed that if the di-alcohol polyester is added to the reaction mixture at the beginning or even after the completion of the transesterification, mixed polyesters are obtained which are relatively brittle despite a high degree of polycondensation, and do not display any advantageous properties apart from their softening point.
In contrast to Schade et al., we have discovered that linear aromatic polyesters having appropriate properties for molding, extruding and solvent casting applications can be obtained by transesterification and polycondensation of mixtures of diaryl esters of dicarboxylic acids, dihydric phenols and an oligomer of a dicarboxylic acid and a diol. High impact polyesters can be produced by the process of this invention.
Accordingly, it is the object of this invention to provide a new method for producing linear aromatic polyesters suitable for molding, extrusion and solvent casting applications and they thereby provide a new and useful linear aromatic polyesters. These and other objects of the invention will become apparent to those skilled in the art from the following detailed description.